Apparatus and Methods Thereof for Power Consumption Measurement at Circuit Breaker Points

ABSTRACT

Apparatus and methods are provided for the measurement of power consumption at points of interest, such as circuit breakers, machines, and the like. Accordingly, means are provided for measurement of power consumption for each electrical sub-network that is controlled by a circuit breaker. Each apparatus is enabled to communicate its respective data, in an environment of a plurality of such apparatuses, to a management unit which is enabled to provide finer granularity power consumption profiles. Challenges of measuring relatively low supply currents, wireless operation in an environment of a large number of apparatuses, and self-powering are addressed.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.12/760,867 filed Apr. 15, 2010, which claims the benefit of U.S.Provisional Patent Application No. 61/169,750 filed Apr. 16, 2009 andU.S. Provisional Patent Application No. 61/272,216 filed Sep. 2, 2009.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to the measurement of power consumptionand more specifically to non-intrusive and self-powered measurement ofelectrical current flow through a power line to enable analysis of powerconsumption on a per circuit breaker basis.

2. Prior Art

In a typical electricity distribution system, power is provided througha main circuit breaker and a device for measurement of the powerconsumption of the entire electrical network connected thereto. However,typically, the main power line is then connected to a plurality ofcircuit breakers, each feeding a smaller section of the electricalnetwork with its specific power requirements. The circuit breaker isadjusted to the amount of maximum current that may be used by thiselectrical sub-network. In industrial and commercial applications,hundreds of such circuit breakers may be installed, each controlling asection of the electrical network. Even in smaller locations, such as ahouse, it is not unusual to find tens of circuit breakers controllingvarious electrical sub-networks.

Non-intrusive measurement of current through a power line conductor haswell known principles. A current transformer (CT) of sorts is createdthat comprises the primary winding as the power line conductor and thesecondary providing an output current inversely proportionate to thenumber of windings. Typically such systems are used for measuringcurrents in very high voltage or current environments, for example, asshown in Gunn et al. in U.S. Pat. No. 7,557,563. These types ofapertures are useful for main power supplies. Using such devices, orpower meters for that matter, is deficient for the purposes of measuringrelatively low currents in an environment of a plurality of circuitbreakers. Providing wireless telemetry on a singular basis, such assuggested by Gunn et al., and other prior art solutions, suffers fromdeficiencies when operating in a noisy environment.

There is a need in the art that is now developing, resulting from themove toward energy conservation to enable analysis of power consumptionon a finer granularity. This would require analysis on at least a percircuit breaker basis and such solutions are not available today. Itwould be further advantageous if a solution may be provided forinstallation in a circuit breaker closet for existing circuit breakers.It would be therefore beneficial to overcome the limitations of theprior art by resolving these deficiencies.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit breaker equipped with a compatible self-poweredpower sensor deployed in accordance with the invention.

FIG. 2 is a block diagram of a first embodiment of a self-powered sensorin accordance with the invention.

FIG. 3 is a circuit diagram of a first embodiment of the analog portionof the self-powered sensor in accordance with the invention.

FIG. 4 is a circuit diagram of a second embodiment of the analog portionof the self-powered sensor in accordance with the invention.

FIG. 5 is a circuit diagram of a third embodiment of the analog portionof the self-powered sensor in accordance with the invention.

FIG. 6 is a schematic diagram of a core with the secondary winding.

FIG. 7 is a schematic diagram of the two parts comprising the core.

FIG. 8 is a schematic diagram of a housing of a self-powered powersensor implemented in accordance with the invention.

FIG. 9 is a flowchart of the operation of a self-powered power sensordeployed in accordance with the invention.

FIG. 10 is a schematic diagram of a system configured in accordance withthe invention.

FIG. 11 is a block diagram of a second embodiment of a self-poweredsensor in accordance with the invention.

FIG. 12 is a circuit diagram of a fourth embodiment of the analogportion of the self-powered sensor in accordance with the invention.

FIG. 13 is a circuit diagram of a fifth embodiment of the analog portionof the self-powered sensor in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Apparatus and methods are provided for the measurement of powerconsumption at points of interest, such as circuit breakers, machinesand the like. Accordingly, means are provided for measurement of powerconsumption for each electrical sub-network that is controlled by acircuit breaker. Each apparatus is enabled to communicate its respectivedata, in an environment of a plurality of such apparatuses, to amanagement unit which is enabled to provide finer granularity powerconsumption profiles. Challenges of measuring relatively low supplycurrents, wireless operation in an environment of a large number ofapparatuses, and self-powering are addressed.

Reference is now made to FIG. 1 where an exemplary and non-limitingsystem 100 is equipped with a compatible self-powered power sensor(SPPS) 110 deployed in accordance with the invention. The SPPS 110 isdesigned to fit either above or below the circuit breaker 120 which isof standard size such that it fits into current circuit breaker closetswithout modification. The SPPS 110 housing is designed, as discussed infurther detail below, to wrap around the power line 130 leading to orgoing out of the circuit breaker 120. The SPPS 110 is designed to enableeasy installation at an existing location or otherwise duringconstruction when the entire electrical network is put in place.

The SPPS contains an electrical circuit the exemplary and non-limingcircuit 200 which is shown in block diagram form in FIG. 2. The circuit200 comprises an analog section 210 that is coupled to a microcontroller220. The analog section comprises a current transformer 212 to transformcurrent from the power line, for example power line 130, to a lowercurrent. The power sensed there from is used for two purposes, the firstis to provide the power needed for the operation of the SPPS 110 and thesecond is to sense the actual power consumption of the load connected tothe power line 130. The current to pulse converter (C2PC) 214 is used togenerate periodically a pulse that is provided to the microcontrollerunit (MCU) 220 and enables the measurement of the power consumption. Themore frequent the pulses the higher the power consumption. The energyharvester 216 stores energy to be used as the power supply for thecircuitry of SPPS 110. It is further enabled to receive a dischargesignal from the microcontroller 220 to enable intentional discharge ofthe energy harvester 216 and prevent overcharge. In one embodiment ofthe invention a Zener diode (not shown) is used to clamp the voltage tothe desired level thereby preventing overcharge.

The circuit 200 further comprises a MCU 220 that is comprised of severalcomponents. An analog-to-digital (A/D) converter 225 that is coupled toa signal processor 224 which is further coupled to the media accesscontrol (MAC) 222 that supports the communication protocol of the SPPS.The MAC 222 provides the data-link layer of the 7 layer standard modelof a communication system. This involves the creation in hardware,software, firmware or combination thereof, of data frames, timing theirtransmission, received signal strength indication (RSSI),acknowledgements, clock synchronization etc. A counter 227 is excited byan interrupt signal received from the analog section 210 and enables thecounting of the number of pulses that, as noted above, is proportionateto the power consumed for a given unit of time. Another A/D converter226 is used to measure the output of the energy harvester 216, and inone embodiment, under control of MCU 220, to cause a discharge thereofas may be needed and as further explained below. In another embodiment,further explained herein below, it can be used to detect that the loadconnected to the measured power line was turned off. A memory 230 iscoupled to the MCU 220 that can be used as scratch pad memory 230 aswell as memory for storage of the plurality of instructions that whenexecuted by the MCU 220 executes the methods discussed herein. Memory230 may comprise random access memory (RAM), read only memory (ROM),non-volatile memory (NVM), other memory types and combinations thereof.

A radio frequency (RF) transceiver 240 is coupled to the MCU 220 and toan antenna 250 to provide one or two-way communication with a managementunit, discussed in more detail below. In one embodiment of the inventionthe RF transceiver 240 supports transmission only, i.e., uplinkcommunication. However, the RF transceiver 240 may comprise a receiverportion to support features such as, and without limitation, sensing fora carrier signal, clock synchronization, acknowledgement, firmwaredownload, and configuration download. Typically, this should be anunlicensed industrial scientific medical (ISM) band transceiver,operative, for example and without limitation, at 2.4 Ghz. In oneembodiment some form of spread-spectrum modulation technique may beused, for example and without limitation, direct sequence spreadspectrum (DSSS), to enable better coexistence with other systems workingin the same environment. The communication rate, discussed in moredetail below, should be high enough to enable coexistence of a couple ofhundred SPPSs in the same electrical closet. The power consumption ofthe RF transceiver 240 should be low enough to adhere with the energyharvesting limitations. Yet another requirement of the RF transceiver240 is to support a communication range sufficient to operate in anelectrical closet, e.g., 3-4 meters metallic reach environment. Inanother embodiment of the invention the range may reach up to a few tensof meters in an indoor environment. This enables the placing of SPPSs onindividual devices, e.g., on machines in a production line of a factory,and a minimum number of bridge units in the area. The RF transceiver 240preferably uses a standard PHY layer supporting, for example and withoutlimitations, IEEE 802.15.4, and/or communication protocol, for exampleand without limitation, Zigbee. Use of such standards enables easyintegration with existing systems that already include wirelesshardware, for example and without limitations, smart meters.

According to the principles of the invention, each time a pulse arrivesfrom the C2PC 214 an interrupt signal is sent to the MCU 220. Responsiveto receiving the interrupt pulse the MCU 220 wakes up and increases thecounter 227 value. The energy stored in each pulse is larger than theenergy required for wakeup and counting, hence enough energy is stillavailable for charging the energy harvester 216 and/or enabletransmission using the RF transceiver 250. The value of the counter 227is proportional to the total charge which went through the primary line130, i.e., current integrated over time. The value in the counter 227,as well as other parameters, are saved in the system's memory 230. TheMCU 220 is enabled to periodically check for a condition to transmit.Such a condition may be one or more of the following conditions:sufficient amount of energy exists, upon a certain time lapse from aprevious transmission, upon collection of certain data such assignificant or otherwise interesting data, and other relevantconditions. According to the principles of the inventions detection ofthe existence of sufficient amount of energy for transmission, forexample, through the A/D converter 226 connected to the energy harvester216, it is possible to detect if its voltage reached a predeterminedvalue.

Upon determination that a transmission is to take place the MCU 220prepares a message to be transmitted. The message is typically a singlepacket of data that may contain various types of information and includethe SPPS's unique identification (UID) which enables a management unitto positively associate the current data received with previous datahandled by the management unit with respect of the SPPS. The value ofcounter 227 value, potentially multiplied by a calibration factorconverting that value into a normalized charge unit relative to othersensors, for example, Ampere-Hour (AH), may be attached as part of thepacket. The calibration factor may be programmed to the SPPS 110 in theNVM of memory 230 during calibration of the circuit 200, as part offinal inspection during manufacturing. This ensures compensation againstinaccuracies typical to the manufacturing process. The calibrationfactor may be a fixed value for all units or a specific calibrationfactor unique to each unit. The later is useful for overcomingproduction tolerances of the SPPS. Other information may include,without limitations, various SPPS status information, hardware version,software version, alerts such as overload, phase information, averagecurrent, temperature, time duration information, power off indication,e.g., upon identification that the load was turned off, and other systemparameters. Such parameters may be saved until such time of transmissionin memory 230, and more specifically in a NVM portion of memory 230. Acyclic redundancy code (CRC) calculation, forward error correction(FEC), and/or data redundancy may be further added to a packet for datavalidation at the receiver side. In one embodiment, when the voltage ofthe harvesting circuitry is determined to be decreasing at a high rate,i.e., the power line load was turned off, the device transmits a messagecontaining the last counter value as no energy may be available untilthe load is switched on again.

When condition(s) to transmit is (are) met, the MCU can implement acarrier sense multiple access (CSMA) mechanism for the purpose ofcollision avoidance. The following steps are therefore taken. First, thereceiver of the RF transceiver 240 is switched on. Second the receiversenses whether there are currently other transmissions. This isparticularly important in the environment in which the SPPS operates,which is an environment rich with SPPSs, possibly a few hundreds ofthem. Third, upon determination that the air is free, the receiver isdisabled and the transmitter of the RF transceiver 240 is enabled fortransmission to send the information message; otherwise, the receiver usdisabled and the circuit 200 is caused to sleep for a random timeinterval, after which the circuit 200 wakes-up and the sequence of stepsis repeated until the desired transmission is completed. In oneembodiment of the invention, after completion of transmission thetransmitter is disabled and the receiver is enabled to receive anacknowledgement signal from the management unit. In another embodimentof the circuit 200 the information messages are short enough and theintervals between transmissions are long enough so that collisions arehighly unlikely. In such an embodiment the transmission of theinformation message may take place without pre-sensing of the air,thereby conserving energy. In yet another embodiment of the invention,after transmission the receiver is activated to receive a clocksynchronization signal. This allows synchronization between the clocksof MCU 220 and the management server 1050 (see FIG. 10), and as furtherexplained herein below.

In yet another embodiment of the invention sufficient amounts of energyare available in the circuit 200 for continuous or longer operation.This is possible in cases where the primary current is above a certainvalue. The MCU 220 can then remain on and perform signal processing onthe non-rectified signal coming directly from the current transformer212. The gathered information may be therefore transmitted morefrequently. This is useful for example for measurements relating to peakvalues, average currents, phase calculation, frequency shiftcalculation, transient and irregular current over short period of time,and total harmonic distortion (THD). The reservoir voltage of energyharvester 216 is constantly measured by means of A/D converter 226 ofMCU 220, in order to prevent overcharge. If necessary a discharge of theenergy harvester 216 is performed through an I/O port. The voltageinformation further provides an indication of the available energy forkeep-alive transmissions when no primary current exists. This may happenwhen the circuit breaker 120 tripped or was otherwise shutdown, orotherwise when no power is consumed by the electrical sub-networkprotected by the circuit breaker 120. In a further embodiment of theinvention a 3-phase SPPS is implemented comprising three analog sections210 each coupled to a single MCU 220, which is further coupled to thetransceiver (240) and an antenna (250). The circuit is configured tohandle three analog sections such that the single MCU 220 can handle theentire operation of a 3-phase SPPS. While a 3-phase SPPS is described itshould be understood that a system comprising a plurality of analogsections maybe implemented, for a single phase or multiple phase SPPS,thereby reducing the costs of such a multi-power-line-sensor SPPS.

Reference is now made to FIG. 3 depicting an exemplary and non-limitingcircuit diagram 300 of a first embodiment of the analog portion 210 ofthe self-powered circuit 200 in accordance with the invention. Theprimary winding of the current transformer 310 is the power line 130 andits AC current induces voltage and current in the current transformer310. The induced current resonates with the resonance capacitor 320 toproduce sufficient voltage to pass through the diode bridge 330. In thecase where Schottky diodes are used this voltage is approximately 0.3V.At the output of the diode bridge a rectified DC current is providedwhich charges the sense capacitor 340 until it reaches a certainthreshold V_(1H). The comparator 360 detects V_(1H) on the sensecapacitor 340, and produces a control signal to the DC/DC controller 370which in turn activates the DC/DC switch 375 and boosts the voltage onthe high capacitance reservoir capacitor 380 to a high voltage V₂,typically up to 12V. The control signal is also used as an interrupt towake up the MCU 220 and raise a counter 227. Each discharge of the sensecapacitor 340 represents a quantum of AH flowing through the maincircuit. The frequency of the pulses is proportional to the primarycurrent and the number of pulses is therefore proportional to the totalAH flowing through the main circuit. The sense capacitor 340 isdischarged through the DC/DC inductor 350 into the reservoir capacitor380. The DC/DC control signal from the DC/DC controller 370 causessuspension of the discharge of the sense capacitor 340, once thecomparator 360 detects a low threshold V_(1L), for example 0.5V, on thesense capacitor 340. The voltage of the reservoir capacitor 380 isregulated by the linear regulator 390 into a steady DC voltage, forexample 3.3V or 2V as the case may be, which is supplied to the MCU 220,RF Transceiver 240, DC/DC controller 370 and the comparator 360.

Upon startup of circuit 300 the reservoir capacitor 380 is charged bythe sense capacitor 340 until enough energy is stored in the reservoircapacitor 380 that provides a sufficient voltage to activate thecomparator 360 and the DC/DC controller 370. The advantages of using aDC/DC converter are twofold: enabling the boosting of the reservoircapacitor 380 into a high voltage, hence enabling an energy reservoirsufficient for many RF transmission cycles; and, enabling a relativelylow V_(1H)/V_(1L) range, hence enabling the circuit 300 to operate atvery low primary currents by producing, typically, only up to 1V at thesense capacitor 380. The voltage of the reservoir capacitor 380 isprovided to the A/D converter 226 of the MCU 220 thereby enabling anintentional discharge to prevent overcharge. Discharge is achieved bythe MCU 220 through control of the I/O terminal of transistor 395. Inanother embodiment, as also previously discussed, a Zener diode (notshown) is used for the purpose of overcharge control. In anotherembodiment the A/D converter 226 is configured to detect if the loadconnected to the primary line was turned off and hence consumes zerocurrent. In this case the voltage on the reservoir capacitor 380 dropsat a high rate as no energy is supplied to the circuit 200. Thetransmitter therefore transmits a single message indicating that powerwas turned off. The message may further contain the last counter valuesampled prior to the reservoir energy depletion. The non-rectifiedoutput of the current transformer 370 is coupled to the A/D converter245 of the MCU 380, for example using a small sense resistor (not shown)thus enabling additional signal processing and measurements when enoughenergy exists in the circuit 300. For example, and without limitations,phase measurement or detection of irregular behavior may be achieved atsuch times. By limiting the voltage of the sense capacitor, the voltageon the CT 310 coil is kept low hence the magnetic core can be operatedbelow its natural saturation point which increases the measurementaccuracy.

The resonance capacitor 320 resonates with the current transformer coilin order to produce a sufficiently large voltage to pass through thediode rectifier. Since the magnetization curve of a typical core is nonlinear at low primary currents, the effective inductance of the corevaries with primary current. In one embodiment of the invention, it isbeneficial to select the resonance capacitor's value so that maximumresonance is achieved at low primary currents. This produces therequired voltage swing to pass through the diode bridge even at very lowprimary currents.

FIG. 4 depicts an exemplary and non-limiting circuit diagram 400 of asecond embodiment of the analog portion 205 of the self-powered sensor110 in accordance with the invention. The circuit is simpler then thecircuit 300 as it does not use a DC/DC controller. In this embodiment,when the sense capacitor 440 reaches 3V, the comparator 450 activatesthe switches 452 and 454. Activation of the switch 452 enables chargingthe reservoir capacitor 470 directly from the sense capacitor 440. Theswitch 454 changes the comparator 450 thresholds. When the sensecapacitor 440 is discharged to 2.2V the comparator disengages thecapacitors, i.e., transfer of energy to the reservoir capacitor 470ceases. The voltage on the reservoir capacitor 470 is regulated to, forexample, 2V, the voltage which is the V_(cc) voltage of the MCU 220 andthe RF transceiver 240. In many cases, the internal voltage regulator ofthe MCU 220 may be used since the voltage range is minimal. When thevoltage of the reservoir capacitor 470 voltage is above, for example,2V, the MCU 220 is capable of waking up and drawing current for pulsecounting and transmission as described above. The MCU 220 enables thereservoir capacitor 470 to be charged to a peak voltage of, for example,2.2V. Overcharge is prevented by intentional discharge as described inthe previous embodiment. In this case, since no DC/DC is used, it iscritical to keep the voltage of the reservoir capacitor 470 lower thanthe low threshold of the sense capacitor 440, for example, 2.2V, inorder to prevent charge from flowing backwards. In another embodiment,as also previously discussed, a Zener diode (not shown) is used for thepurpose of overcharge control. An optional small auxiliary battery 460is used in order to feed the comparator 450, provide initial operatingenergy when the reservoir capacitor 470 is not fully charged, andprovide enough energy for low frequency, for example once per day,keep-alive transmissions when no primary current exists. Keep alivetransmissions are important in order to notify the system of theexistence of the sensor even when no primary current exists.

FIG. 5 depicts an exemplary and non-limiting circuit diagram 500 of athird embodiment of the analog portion 205 of the self-powered sensor110 in accordance with the invention. In this embodiment of the analogportion 205 there is only one large sense capacitor 540 and no reservoircapacitor nor a DC/DC controller. The reason for using lesser componentsin the circuits shown in FIGS. 4 and 5 is to reduce the component countand thereby reduce the bill-of-materials (BOM) of the solution. In thecircuit 500 the sense capacitor 540 also functions as the energy sourcefor, typically, a single transmission. Therefore, the sense capacitor540 of this embodiment is designed with a rather large capacitance, forexample 1 mF. According to the principles of operation of the circuit500 the comparator 550 detects when the sense capacitor 540 is charged,for example, up to 4V, and opens the switch 552 towards the linearregulator 570. The linear regulator 570 provides a regulated voltage,for example a 3V output, thereby allowing the MCU 220 to draw currentresulting in discharge of the sense capacitor 540. Due to the activationof switch 554, discharge to a lower reference voltage, for example 3V,is detected by the comparator 550 and discharge is stopped. The MCU 220is enabled to perform operations which discharge the sense capacitor 540to perform the counting operation and transmission when needed. The MCU220 is further enabled to measure the voltage of the sense capacitor anddischarges it down to a lower voltage, for example 3V, intentionallywhen performing operations that do not consume the entire energy. Anoptional battery 560 is used to provide a reference voltage to thecomparator 550, as well as to allow keep-alive transmissions when theprimary current is below a minimum detectable current. In anotherembodiment, as also previously discussed, a Zener diode (not shown) isused for the purpose of overcharge control. In another embodiment, asalso previously discussed, a linear regulator is not used and the MCU'sinternal regulator regulates the input voltage.

In another embodiment of the invention, power measurement is done bymeasuring the voltage change rate on the sense capacitor, e.g.,capacitors 540, 440 or 340. The sense capacitor voltage is measured byA/D 226. The MCU 220 then lets the capacitor discharge through aresistor, for example resistor 395, for a fixed period of time, duringwhich the MCU 220 can be set to a low power mode. The voltage level ofthe sense capacitor is measured after the elapse of the fixed period oftime, and the voltage difference (ΔV) between the two measurements iscalculated. ΔV consists of a negative fixed part, i.e., the voltagedischarge through resistor 395, plus a positive variable partproportionate to the charge rate of the capacitor due to the primarycurrent flow.

Key to the operation of the SPPS 110 is that it is capable of addressingseveral critical challenges to its successful operation. Three keyissues are the minimum power detection of the current transformer 212,the power balance of the circuit 200, and wireless coexistence in anenvironment of a plurality of SPPSs 110 that may include severalhundreds of SPPSs. In order for an SPPS 110 to be a useful device it isnecessary that it be capable of detecting as low as possible currentsflowing through the primary lead 130. The design must take intoconsideration the limited space typically available for an apparatussuch as, but not limited to, SPPS 110 that must fit dimensionrestrictions of the circuit breaker 120. In other embodiments of theinvention other size restrictions may apply, however these should not beviewed as limiting the scope of the invention. Inductance of thesecondary winding is approximately:

$L = \frac{\mu_{0}\mu_{r}N^{2}A}{1}$

Where N is the number of windings, λ_(r) is the relative permeability ofthe magnetic material, such as, and not limited to, strip wound iron, μ₀is the permeability of free space, A is the cross section of the core,further discussed with respect of FIGS. 6 and 7 below, and 1 is theeffective length of the core. For N=1500, μ_(r)=1000, μ₀=4π10⁻⁷, A=40mm², and 1=20 mm, the inductance is L=5.5 Hy. The current ratio betweenthe secondary current I_(s) and the primary current I_(p) isapproximately, for an ideal transformer, I_(p)/I_(s)=N. The voltage onthe secondary coil is given by V_(s)=I_(s)ωL=I_(p)ωL/N, and at f=50 Hzω=2πf=314 rad/sec. Therefore, V_(s)=I_(p)ωL/N=1.15I_(p). Assuming a 1Vdrop over the diode rectifier, for example diode rectifier 330, andcharge voltage of 1V then at least 2V are needed in order for the systemto operate. Hence, there is a minimum detectable current of 2/1.15=1.7 Apeak=1.2 A RMS. Using the resonance capacitor, for example resonancecapacitor 320, the impedance is decreased by a factor of 1/(X_(L)−X_(C))where X_(L) is the impedance of the core and X_(C) Is the impedance ofthe resonance capacitor. Taking an accumulative tolerance of ±20% forthe capacitance and inductance, results in a worst case of 40% increasein signal, and hence the minimum detectable current is, in thisexemplary case, 1.2×0.4=0.48 A, which represents a minimum detectablepower of 105 VA at 220V. At 110V 60 Hz, the minimum detectable currentin the exemplary case is ⅚×0.48=0.4 A and a minimum detectable power of44 VA. Since L is proportional to N² and to A and V is proportional to1/N, the minimum detectable current may be decreased by increasingeither N or A. However, it is essential to ensure that the entire core,and its respective secondary winding, fit in the size constraints ofSPPS 110, and an increase of N or A may have a material effect thereon.

Furthermore, to make the SPPS 110 an operative device it is essential toensure that a sufficient amount of power may be made available throughthe operation of the circuits discussed hereinabove. Following is anexemplary and non-limiting analysis thereof. Firstly it is essential tounderstand the energy requirements of each of the key components: thetransmission cycle, the counting cycle and the logic operation. Failureto address these issues may result in non-operative circuits. In allcases the assumption is for a 3V operation. For the transmission cycle atransmission current of 20 mA is used for a period of 5 mSec. Aprocessing current of 1 mA is used during a 10 mSec period of wakeup andprocessing. Therefore the total energy requirements for the transmissioncycle is: 3V×(20 mA×5 msec+1 mA×10 msec)=0.33 mJ. For the counting cyclea processing current of 1 mA is used for a wakeup and processing periodof 5 mSec. Therefore the energy requirements for this counting cycleare: 3V×1 mA×5 msec=15 μJ. Lastly, the logic operation requires acontinuous current of 50 μA, resulting in a continuous power consumptionof: 3V×0.05 mA=150 μW. The total energy has to be supplied reliably bythe power supply circuit, for example, circuit 300. It is thereforenecessary that the sense capacitor, for example sense capacitor 340, andthe reservoir capacitor, for example reservoir capacitor 380, providesufficient energy for the performance of the desired operations. Theabove assumptions are typical for common low power MCUs and radiofrequency integrated circuits (RFICs).

To address the energy balance of the circuit 200 it is necessary toensure that the sense capacitor, for example sense capacitor 320, iscapable of supplying sufficient energy for the counting cycle and thatthe reservoir capacitor, for example reservoir capacitor 380, is capableof supplying enough energy for several transmission cycles. Both areaddressed in the following exemplary and non-limiting calculations. Ifthe sense capacitor C₁ is equal to 1 mF and is charged to V₁=1V anddischarged to V₂=0.5V, then the total discharge energy is: E=0.5C₁×(V₁²−V₂ ²)=3750. It has been shown hereinabove that the counting cyclerequires 15 μJ which is less than 3% of the available energy. Theremaining energy is accumulated for the purposes of transmission, forexample, in the reservoir capacitor. Assuming a reservoir capacitor, forexample capacitor 320, having a value of 0.375 mF, the capacitor beingcharged to V₁=5V and discharged to V₂=3V, then the total energy is:E=0.5C₂×(V₁ ²−V₂ ²)=3 mJ. A previous calculation has shown that thetransmission cycle consumes around 0.33 mJ and hence roughly ninetransmission cycles are possible under these conditions. Now it ispossible to determine the number of counting cycles it takes to chargethe reservoir capacitor with the required amount of energy. Theavailable energy is 3600 and with a 50% DC/DC controller efficiencythere are 1800 at every sense capacitor pulse. By dividing the amount ofenergy required for several transmission cycles, e.g., 3 mJ, by theamount of energy charged each cycle, e.g., 0.18 mJ, it is determinedthat approximately 17 sense capacitor cycles are needed to charge thereservoir capacitor with the required energy.

In order to ensure proper operation of the circuit 200 it is necessaryto ensure a positive energy balance for continuous system operation evenat the lowest primary currents. It is therefore necessary to calculatethe power in to the system versus the power out of the system, the laterhaving to be smaller than the earlier. For the power in, at a primarycurrent I_(p)=0.5 A and N=1500 results in a secondary current ofI_(s)=0.33 mA. Using the same figures as above, i.e., a sense capacitorof 1 mF, discharge voltage down to 0.5V and charge voltage of 1V, thecharge time is T=CΔV/I_(s)=1.5 Sec. The available energy of 375 μJtherefore provides 375/1.5=0.25 mW. Assuming 80% DC/DC efficiency, theavailable power in is 200 μW. The power out is a combination of thecontinuous logic operation, the counting process and the transmissions.The continuous logic operation requires 150 μW as shown above. Thecounting processing requires 15 μJ for a period of 375 mSec which isequivalent to 40 μW. Assuming a transmission once every one minute then360 μJ are required every 60 seconds which are 6 μW. The total powerconsumption is therefore 196 μW which is less than the 200 μW availableas explained herein above. It should be noted that a higher primarycurrent results in an improved power balance that enables an increase ofthe transmission frequency, performing continuous signal processing,storing energy for times when no primary current exists, andcombinations thereof.

FIGS. 6 and 7 show schematic diagrams 600 and 700 of a core with thesecondary winding and the core separated into two parts. The core iscomprised of two parts 610 and 620 that are separable from each other,however, as shown in FIG. 7, are designed so as to ensure that when theyare assembled they provide good magnetic flow through the core byreducing the air-gap between the two parts to minimum, for example 10μm. While an exemplary shape of the two portions of the core is shownthese are merely for explanation purposes and other designs are possibleto achieve the required results. It is essential, as explained hereinabove, that the core fit in the dimensions allotted in the SPPS 110 sothat it can properly fit in an electricity closet in conjunction with acircuit breaker. The secondary windings 630 of the current transformer212 are wound on one of the sections of the core, for example, section610 which is the stationary section that is placed in the exemplary andnon-limiting housing 800 shown with respect of FIG. 8. In this example,these may be two windings connected in series, of two independentsecondary windings (see FIG. 6). The moveable section of the core, forexample section 620, is placed in section 810 of the housing 800 whichis separable from section 820 of the housing, in which section 610 isplaced. When separating section 810 from section 820 it is possible toplace them around power line 130 so that when the sections 810 and 820are reconnected the power line 130 is placed within the core perimeterthereby completing the current transformer 212. Each SPPS 110 isassigned a unique identification (ID), for example a MAC address thatmaybe 16 bytes in length, that is placed on the housing 800 at, forexample, location 840. At installation of the SPPS the MAC address isread by a technician installing the system for configuration purposes.In one embodiment machine readable code is provided, e.g., barcode, toenable automatic reading using a reader. While a core comprising of twosections is described hereinabove, it should be noted that otherimplementations for a core are possible without departing from the scopeof the invention. In one embodiment a single section core is used and insuch a case the primary line must be inserted through the hole in thecore. It may require disconnection of the line and threading it throughthe core for mounting the SPPS device.

An exemplary and non-limiting flowchart 900 depicted in FIG. 9 describesthe operation of a SPPS deployed in accordance with the invention. InS910 the SPPS, for example, SPPS 110, checks if counting pulse wasreceived and if so execution continues with S920; otherwise, executioncontinues with S910. In S920 a count is performed in accordance with theprinciples described herein above, which may include the discharge ofthe sense capacitor, for example capacitor 320. In S930 it is checkedwhether there is sufficient energy to perform a transmission and is soexecution continues with S940; otherwise, execution continues with S910.In S940 it is checked whether it is time to transmit by the SPPS 110 andif so execution continues with S950; otherwise, execution continues withS910. In S950 SPPS 110 senses the environment for another transmissionto avoid transmission collisions as discussed herein above. In S960 itis checked if it is possible to transmit and if so execution continueswith S980; otherwise, in S970 a random wait period is determined andexecution then continues with S930. In S980 the information gathered bythe SPPS 110 is transmitted, the information transmitted contains dataas discussed herein above. In S990 it is checked whether the operationshould continue and if so execution continues with S910; otherwise,execution terminates. An optional step may be added after transmissionis complete for the purpose of reception of feedback information fromthe unit receiving the information sent by the transmitter. Suchfeedback information may include, but is not limited to, acknowledgeinformation and/or synchronization information.

Reference is now made to FIG. 10 where an exemplary and non-limitingsystem 1000, configured in accordance with the principles of theinvention, is shown. The system comprises a plurality of SPPS 1010communicatively coupled to a communication link 1020. The SPPS 1010 maybe placed in an electrical closet before or after respective circuitbreakers or, at the input to specific power consuming units. Themanagement server is equipped with a transceiver enabling thecommunication with the plurality of SPPS 1010 using one or more of thecommunication schemes discussed herein above. The communication bridge1020 is configured to communicate with those SPPSs 1010 it is configuredto operate with, using for identification their respective MACaddresses. The communication bridge 1020 is coupled to a network 1020which may be, but is not limited to, a local area network (LAN), a widearea network (WAN), a metro area network (MAN), the Internet, the worldwide web (WWW), the likes and combinations thereof. The communicationlink can be, but is not limited to, a WLAN (Wireless LAN), for example802.11 also known as WiFi, a wireless sensor area network, for example802.15.4 also known as Zigbee, power line communication (PLC), or acellular to modem network such as GPRS or CDMA. In one embodiment of theinvention the communication bridge aggregates the data from theplurality of sensors 1010-1 to 1010-N prior to sending it to thenetwork. To the network there are coupled a database 1040 to accumulatedata collected by the communication bridge 1020. The communicationbridge 1020 may be placed in each closet and aggregate a plurality ofSPPS 110 communications. In one embodiment the communication bridge 1020is responsible for the phase calculation discussed in more detail hereinbelow. Further coupled to the network is a management server 1050 thatbased on the data accumulated in database 1040 may provide a client 1060processed information respective of the collected data as well ascommunicate with other application software, for example buildingmanagement systems (BMSs). In one embodiment of the invention theminimum number of winding in the secondary coil is 500.

In one embodiment of the invention the communication bridge 1020 isenabled to provide information with respect to a phase and enable thesystem to calculate a phase shift. Knowledge of the phase shift betweencurrent and voltage is used to calculate the power factor (cos φ), hencedetermine more accurately the real active power flowing through thepower line. When it is determined that there is sufficient energy inenergy reservoir 216 then MCU 220 may become operative in continuousmode, for as long as such sufficient energy is available, or untiloperation is complete. Using AD converter 225 MCU 220 detects the peakcurrent of the current transformer 212. The time of the peak relative toa clock synchronized between the sensor and the bridge unit is recordedand, when appropriate, transmitted to the communication bridge 1020,according to the principles discussed hereinabove. communication bridge1020 is further enabled to detect the peak of the power supply voltagenearest to the sensors by at least a peak detector (not shown) coupledto the communication bridge 1020 and to a reference power line. The timeof the peak of is recorded by the communication bridge 1020continuously. As the clocks of the communication bridge 1020 and circuit200 are synchronized, as further discussed hereinabove, it is nowpossible for the communication bridge 1020, upon receiving informationfrom the circuit 200 respective of the measure peak and time, todetermine the phase shift between the reference power line voltage andthe current measurement made by the circuit 200. It should be noted thatthe use of a peak detector enables the system to become agnostic to thedifferences in the utility grid frequency, e.g., 60 Hz for the USAversus 50 Hz in Europe, as well as to any other error or change in thesupply voltage frequency.

Reference is now made to FIG. 11 where an exemplary and non-limitingsecond embodiment of a SPPS 1100 is shown. A key difference may beobserved in the microcontroller 220 that does not receive a pulse as aninterrupt signal as was shown in the previously described embodiments,for example in FIG. 2. Similar components to those of FIG. 2 are notfurther discussed herein, unless necessary for clarity. The notablechange is in the analog section 1110 that comprises a currenttransformer 212, an energy harvester 216, a switch 1114 and a senseresistor 1112. In normal operation the switch 1114 is positioned toenable energy harvesting by the energy harvester 216. Periodically, forexample under the control of the microcontroller 220, the switch 1114 isactivated to short the secondary winding of transformer 212 through thesense resistor 1112, typically having a low resistance. The voltage onthe sense resistor 1112 is sampled by the ADC 225. In order for thesystem 1100 to identify a voltage peak the process is repeated severaltimes in each cycle. The switch 1114 is toggled between the twopositions to enable energy harvesting most of the time in a firstposition, and measurement of the voltage periodically when in the secondposition. The sampling is averaged over a number of cycles and dividedby the resistance value of the sense resistor 1112 to provide thecurrent value. The current value is then multiplied by a time intervalto obtain the total charge value, for example, in Ampere Hours. Acalibration factor, as discussed herein above, can also be used withrespect of system 1100.

The analog section may be implemented as shown in the exemplary andnon-limiting circuit diagram 1200 of FIG. 12. Normally, the switches1210 and 1220, connected between the resonance capacitor 320 and thebridge rectifier 330 are off, so that the harvesting capacitor 380 ischarged. The voltage of the harvesting capacitor 380 is limited to avoidovercharge as discussed in detail herein above with respect to otherembodiments of the invention. From an energy harvesting point of view,FIG. 12 represents an embodiment close to the one shown in FIG. 5 butembodiments similar to the ones shown in FIGS. 3 and 4, in terms of theharvesting circuitry, are also possible. To perform a measurement themicrocontroller 220 switches the transistors 1210 and 1220 using theirrespective I/O ports. According to the principles of the inventionswitches 1210 and 12220 are operated simultaneously in opposite phases.Although measurement is preformed on a single resistor 300 rather thantwo, the use of the two switches and two resistors is in order toprevent DC load on the transformer 212. This is required to avoidsaturation and distortion of the measurement results. It would beappreciated by those skilled in the art that one switch conducts in thepositive part of the cycle, and the other switch conducts in thenegative part of the cycle. It should be noted however that topologiesusing a single switch which can symmetrically conduct in both directionsare possible, for example, by using a pair of MOSFET transistorsconnected in series. When the switches are active the current flowsthrough the appropriate sense resistor instead of charging theharvesting capacitor 380. According to the invention, the senseresistors have a low impedance relative to the self resistance of thetransformer coil. This enables a close to short circuit current flow,keeping the voltage across the resistor low enough thus maintainingminimal flux across the core and avoiding saturation of the transformer212. In one embodiment of the invention, after switching on the senseresistors, the MCU 220 waits a certain time interval, typically a coupleof hundreds of milliseconds, or switch to an off/power save mode, beforeperforming the measurement, in order to allow for the resonancecapacitor to discharge. This ensures high accuracy and better linearityof the measurement results. In accordance with the principles of theinvention, in cases where it is possible to use two coils, a firstsecondary coil used to measure the voltage using the ADC 225 while thesecond secondary coil (see prior descriptions of FIGS. 6 and 8) is usedfor the purpose of energy harvesting, thereby eliminating the need forswitching at the expense of a potential increase in size of the SPPS.The value of the sense resistor may be easily calculated. Assuming theSPPS is designed for a maximum primary current of 30 A then with N=1000the maximum short circuit current of the secondary winding would be 30mA. If the maximum input to the ADC 225 is 1V then the sense resistor1112 is to be 30Ω. The resistance of a thin, e.g., 0.1 mm, copper wirewith 1000 windings at typical dimensions of the SPPS is approximately100Ω. Referring to the energy balance calculation explained hereinabovewith respect to different embodiments, a similar amount of energycalculated before for the purpose of pulse counting, can be used herefor the purpose of A/D activation and sampling, thus this embodimentdoes not significantly differ from the previous ones in terms of energyconsumption. Therefore a sufficient amount of energy is available forproper system operation even when a very low primary current exists.

In yet another exemplary embodiment of the analog section circuit 1300,shown in FIG. 13, a voltage doubler 1340 is used. In fact, the bridgerectifier described herein above with respect to all of the otherembodiments can be replaced by a voltage multiplier. A person skilled inthe art would readily note that the voltage multiplier may be a voltagedoubler, tripler, quadrupler or any other type of passive voltagemultiplier topology, without departing from the scope of the invention.The exemplary and non-limiting circuit 1300 shows a simpleimplementation of a voltage doubler 1340. The voltage on the harvestingcapacitor 380 is double the voltage on the transformer 310 afterresonance. In some cases the use of a voltage multiplier is advantageousat the lower current range. Also, specifically referring to the senseresistor topology, the voltage multiplier simplifies the grounding ofthe circuit as a common ground can be connected to the harvestingcapacitor and the sensing resistor, whereas when using the bridgerectifier a differential voltage measurement needs to be made.

The principles of the invention, wherever applicable, are implemented ashardware, firmware, software or any combination thereof. Moreover, thesoftware is preferably implemented as an application program tangiblyembodied on a program storage unit or computer readable medium. Theapplication program may be uploaded to, and executed by, a machinecomprising any suitable architecture. Preferably, the machine isimplemented on a computer platform having hardware such as one or morecentral processing units (“CPUs”), a memory, and input/outputinterfaces. The computer platform may also include an operating systemand microinstruction code. The various processes and functions describedherein may be either part of the microinstruction code or part of theapplication program, or any combination thereof, which may be executedby a CPU, whether or not such computer or processor is explicitly shown.In addition, various other peripheral units may be connected to thecomputer platform such as an additional data storage unit and a printingunit. The circuits described hereinabove may be implemented in a varietyof manufacturing technologies well known in the industry including butnot limited to integrated circuits (ICs) and discrete components thatare mounted using surface mount technologies (SMT), and othertechnologies. The scope of the invention should not be viewed as limitedby the types of packaging and physical implementation of the SPPS 110 orthe communication bridge 1020.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the principlesof the invention and the concepts contributed by the inventor tofurthering the art, and are to be construed as being without limitationto such specifically recited examples and conditions. Moreover, allstatements herein reciting principles, aspects, and embodiments of theinvention, as well as specific examples thereof, are intended toencompass both structural and functional equivalents thereof.Additionally, it is intended that such equivalents include bothcurrently known equivalents as well as equivalents developed in thefuture, i.e., any elements developed that perform the same function,regardless of structure.

What is claimed is:
 1. An apparatus comprising: at least one analogsection having a current transformer having a transformer coreconfigured to mount around an alternating current (AC) power line makingit a primary winding of the current transformer, the analog sectionharvesting energy from a secondary winding wound around the transformercore, and storing harvested energy for use by components of theapparatus, the analog section providing an analog signal responsive toAC in the power line and a pulse having a frequency responsive to thealternating current in the AC power line; a microcontroller coupled tothe at least one analog section to receive energy for the operation ofat least the microcontroller, and to receive the analog signal and thepulses; a memory coupled to the microcontroller; and a transmittercoupled to the microcontroller to transmit, under the control of themicrocontroller, information responsive to at least one of the analogsignal and the pulses as an indication of the power consumption of aload connected to the power line.
 2. The apparatus of claim 1, furthercomprising: a resonance capacitor coupled in parallel to the secondarywinding of the current transformer.
 3. The apparatus of claim 1, furthercomprising one of: a diode bridge coupled in parallel to the secondarywinding of the current transformer; or a voltage multiplier coupled inparallel to the secondary winding of the current transformer.
 4. Theapparatus of claim 1, wherein the transformer core comprises a firstsection having the secondary winding wound thereon, the first sectionand the second section fitting together such that the power line issurrounded by the transformer core by the first section and secondsection to achieve an essentially uninterrupted magnetic path around thepower line.
 5. The apparatus of claim 1, wherein the number of turns inthe secondary winding is greater than
 500. 6. The apparatus of claim 1,further comprising, in the analog section, a sense capacitor andcircuitry for periodically discharging the sense capacitor forprevention of saturation of the core.
 7. The apparatus of claim 1,wherein the microcontroller operates in a continuous mode whensufficient energy is available from the analog section, and in thecontinuous mode, performs continuous measurements of the analog signal,stores information responsive to the continuous measurements and causethe transmission of the information using the transmitter.
 8. Theapparatus of claim 7, wherein the continuous measurements include atleast one of: peak detection, phase detection, power disruption.
 9. Theapparatus of claim 1, further comprising a receiver coupled to themicrocontroller.
 10. The apparatus of claim 9, wherein the receiver isenabled to perform at least one of: sense for a carrier signal, receivean acknowledge signal, receive a synchronization information.
 11. Theapparatus of claim 1, wherein the memory contains calibrationinformation of the apparatus.
 12. A system for power managementcomprising: at least one self-powered power sensor (SPPS) coupled aroundan alternating current (AC) power line; a communication bridge adaptedto communicate with the at least a SPPS and further coupled to anetwork; a management server coupled to the network and adapted toreceive information from the communication link respective of the atleast one SPPS; and a database coupled to the network for storing atleast the information; the SPPS having at least one analog sectioncomprising a current transformer comprising a transformer coreconfigured to mount around an alternating current (AC) power line makingit a primary winding of the current transformer, the analog sectionharvesting energy from a secondary winding wound the transformer core,and storing it for use by components of the apparatus and periodicallyproviding a pulse having a frequency responsive to the alternatingcurrent in the AC power line; a microcontroller coupled to the at leastone analog section to receive harvested energy, to receive at least ananalog signal responsive to the alternating current in the AC powerline, and the pulse; a memory coupled to the microcontroller; and, atransmitter enabled to periodically transmit, under the control of themicrocontroller, information respective of the power consumption of thepower line.
 13. The system of claim 12, further comprising: a clientnode coupled to the network and enabled to display at least informationrespective of the power consumed through the AC power line associatedwith the at least one SPPS.
 14. A method for sensing power consumptioncomprising: receiving a pulse from a current to pulse converter of ananalog section of a self-powered power sensor (SPPS); counting thenumber of pulses received; activating a transmitter of the SPPS fortransmission; transmitting information respective of the number ofpulses counted upon determination that the SPPS has accumulatedsufficient power for transmission; and deactivating the transmitter ofthe SPPS; the SPPS having at least one analog section comprising acurrent transformer comprising a transformer core configured to mountaround an alternating current (AC) power line making it a primarywinding of the current transformer, the analog section harvesting energyfrom a secondary winding wound around the transformer core, and storingit for use by components of the apparatus and periodically providing apulse having a frequency responsive to the alternating current in the ACpower line; a microcontroller coupled to the at least one analog sectionto receive harvested energy, to receive at least an analog signalresponsive to the alternating current in the AC power line, and thepulse; a memory coupled to the microcontroller; and, a transmitterenabled to periodically transmit, under the control of themicrocontroller, information respective of the power consumption of thepower line.
 15. The method of claim 14, further comprising: activating areceiver of the SPPS to sense another transmission carrier signal;enabling the transmission of the information if a receiver of the SPPSdoes not detect another transmission carrier signal; and deactivatingthe receiver.
 16. The method of claim 15, further comprising: receivinga signal from a source.
 17. The method of claim 16, wherein the signalcomprises at least one of: an acknowledge signal, synchronizationinformation.
 18. A method for sensing power consumption comprising:receiving a first signal responsive of a primary current sensed bysensing resistor of an analog section of a self-powered power sensor(SPPS); activating a transmitter of the SPPS for transmission;transmitting information respective of the first signal upondetermination that the SPPS has accumulated sufficient power fortransmission; and deactivating the transmitter of the SPPS; the SPPShaving at least one analog section comprising a current transformercomprising a transformer core configured to mount around an alternatingcurrent (AC) power line making it a primary winding of the currenttransformer, the analog section harvesting energy from a secondarywinding wound around the transformer core, and storing it for use bycomponents of the apparatus and periodically providing a pulse having afrequency responsive to a current in the AC power line; amicrocontroller coupled to the at least one analog section to receiveharvested energy, to receive at least an analog signal responsive to thecurrent in the AC power line, and the pulse; a memory coupled to themicrocontroller; and, a transmitter enabled to periodically transmit,under the control of the microcontroller, information respective of thepower consumption of the power line.
 19. The method of claim 18, furthercomprising: activating a receiver associated with the SPPS to senseanother transmission carrier signal; enabling the transmission of theinformation if a receiver of the SPPS does not detect anothertransmission carrier signal; and deactivating the receiver.
 20. Themethod of claim 19, further comprising: receiving a second signal from asource.
 21. The method of claim 20, wherein the second signal comprisesat least one of: an acknowledge signal, synchronization information. 22.An apparatus comprising: at least one analog section comprising acurrent transformer comprising a transformer core configured to mountaround an alternating current (AC) power line making it a primarywinding of the current transformer, the analog section for harvestingenergy from a secondary winding wound the transformer core, and storingit for use by components of the apparatus and periodically switching toallow sampling of a current flowing through the current transformer by asense resistor; a microcontroller coupled to the at least one analogsection to receive harvested energy, at least an analog signalresponsive to the alternating current in the AC power line by the senseresistor; a memory coupled to the microcontroller; and a transmitterenabled to periodically transmit, under the control of themicrocontroller, information responsive to the power consumption of aload connected to the power line.
 23. The apparatus of claim 22, whereinthe switching comprises use of a first switch operative in the positivephase of the AC cycle and a second switch operative in the negativephase of the AC cycle.
 24. The apparatus of claim 22, wherein the senseresistor is coupled to a secondary winding of the current transformer,and has a resistance which is smaller than the resistance of thesecondary winding.
 25. An apparatus comprising: at least one analogsection comprising a current transformer comprising a transformer coreconfigured to mount around an alternating current (AC) power line makingit a primary winding of the current transformer, the analog sectionharvesting energy from a secondary winding wound around the transformercore, and storing it for use by components of the apparatus using afirst secondary winding of the current transformer and a sense resistorcoupled to a second secondary winding for sensing the current of thetransformer; a microcontroller coupled to the at least one analogsection to receive harvested energy and at least an analog signalresponsive to the alternating current in the AC power line by the senseresistor; a memory coupled to the microcontroller; and a transmitterenabled to periodically transmit, under the control of themicrocontroller, information respective of the power consumption of aload connected to the power line.
 26. The apparatus of claim 25, whereinthe sense resistor has a resistance which is smaller than the resistanceof the second secondary winding of the current transformer.
 27. Theapparatus of claim 25, further comprising: a resonance capacitor coupledin parallel to the secondary winding of the current transformer.
 28. Theapparatus of claim 25, further comprising one of: a diode bridge coupledin parallel to the secondary winding of the current transformer; or avoltage multiplier coupled in parallel to the secondary winding of thecurrent transformer.